Red Llama with State Variable Filter Tone Control

[Zwachi]

Hi guys,

I thought about tone controls for a Red Llama circuit and came up with this idea. I found a CMOS version of a SVF in the MXR Envelope Filter which needs 3 inverter. The left over one could be used as a summing amp for highpass, bandpass and lowpass outputs to have a very flexible 3 band tone control without too much efforts.
Economical every inverter of the CD4069UBE is uesd.

Now here are my question marks..
Do I have to add coupling caps between the tandem pots and the inverters? (MXR Filter has a CD4066 with Mosfet switches that may decouple?)
How about the noise level from all inverters together? Is there a chance to get anything but noise?

The UBE Screamer from ROG is quite close but only has a lowpass output..


Looking forward for your experiences!
Thanks!

[ElectricDruid]

The UBE screamer isn't quite close, since it doesn't have the SVF structure. It uses the same CMOS chip instead of op-amps, but that's about it.

Building an SVF around a CMOS chip as the integrators is a new one on me, and it might even work, but I have to say that I'd be doubtful. I'm digging the idea though, so I hope you can get it up and running successfully. I've built many SVF circuits and they tend to be pretty fussy, particularly about phase shifts in the integrator stages. Maybe CMOS stages repurposed as cr*p op-amps make that better or maybe they don't, I dunno. The typical effect you see is that the resulting filter has much more resonance at high frequencies than low ones. If you're just looking to use the SVF as a low/mid/high tone control, you probably avoid that problem since you don't want any resonance (phew! bullet dodged!).

Without having tried it, I'd say your schematic looks feasible and I'd give it whirl. There's all kinds of things that could go wrong, but at the same time, you're moving away from most of the obvious disasters and it might just work straight away.

Good luck!
Tom